There are a few reasons that relays might be uploading desciptors
without saying X-Desc-Gen-Reason:
1. They are running an old version of our software, before 0.3.2.stable.
2. They are not running our software, but they are claiming they
are.
3. They are uploading through a proxy that strips X-Desc-Gen-Reason.
4. They somehow had a bug in their software.
According to the 25686 data, 1 is the most common reason. This
ticket is an attempt to diagnose case 4, or prove that case 4
doesn't actually happen.
* REFACTORS `UnvalidatedProtoEntry::from_str` to place the bulk of the
splitting/parsing logic in to a new
`UnvalidatedProtoEntry::parse_protocol_and_version_str()` method (so that
both `from_str()` and `from_str_any_len()` can call it.)
* ADD a new `UnvalidatedProtoEntry::from_str_any_len()` method in order to
maintain compatibility with consensus methods older than 29.
* ADD a limit on the number of characters in a protocol name.
* FIXES part of #25517: https://bugs.torproject.org/25517
In protover.c, the `expand_protocol_list()` function expands a `smartlist_t` of
`proto_entry_t`s to their protocol name concatenated with each version number.
For example, given a `proto_entry_t` like so:
proto_entry_t *proto = tor_malloc(sizeof(proto_entry_t));
proto_range_t *range = tor_malloc_zero(sizeof(proto_range_t));
proto->name = tor_strdup("DoSaaaaaaaaaaaaaaaaaaaaaa[19KB]aaa");
proto->ranges = smartlist_new();
range->low = 1;
range->high = 65536;
smartlist_add(proto->ranges, range);
(Where `[19KB]` is roughly 19KB of `"a"` bytes.) This would expand in
`expand_protocol_list()` to a `smartlist_t` containing 65536 copies of the
string, e.g.:
"DoSaaaaaaaaaaaaaaaaaaaaaa[19KB]aaa=1"
"DoSaaaaaaaaaaaaaaaaaaaaaa[19KB]aaa=2"
[…]
"DoSaaaaaaaaaaaaaaaaaaaaaa[19KB]aaa=65535"
Thus constituting a potential resource exhaustion attack.
The Rust implementation is not subject to this attack, because it instead
expands the above string into a `HashMap<String, HashSet<u32>` prior to #24031,
and a `HashMap<UnvalidatedProtocol, ProtoSet>` after). Neither Rust version is
subject to this attack, because it only stores the `String` once per protocol.
(Although a related, but apparently of too minor impact to be usable, DoS bug
has been fixed in #24031. [0])
[0]: https://bugs.torproject.org/24031
* ADDS hard limit on protocol name lengths in protover.c and checks in
parse_single_entry() and expand_protocol_list().
* ADDS tests to ensure the bug is caught.
* FIXES#25517: https://bugs.torproject.org/25517
